The invention relates to Orthogonal Frequency Division Multiplexing (OFDM) systems, and is more particularly related to transmitting and receiving digital information within a Frequency Modulated (FM) signal band using OFDM.
As is well-known, the center frequencies of adjacent FM bands/channels are separated by 200 kHz. As is also well-known, analog signals within the audio band are typically frequency modulated onto a FM carrier for transmission to a receiver. It is now possible to also transmit information (e.g., analog/digital information) in the upper and lower side bands of a FM carrier signal. This is broadly illustrated in FIG. 1, in which transmitter 105 transmits via antenna 125 an analog host signal supplied by source 110 over a FM carrier (represented by signal a1). It also transmits digital versions of signals supplied by source 115 in the upper and lower side bands (represented by side band signals b1 and b2), respectively, of the FM carrier. We have recognized that it is likely that the host signal, a2, as well as the first adjacent FM signals, c1 and c2, will interfere with the side band signals (represented by the shaded portions of signals b3 and b4) during the transmission of the FM signal from antenna 125 to a receiver antenna 205. Disadvantageously, such interference makes it extremely difficult for receiver 200 to recover the signal transmitted in the side bands, since the inference sorely corrupts the side band signal.
We have recognized that such interference may be sufficiently minimized by filtering the side band signal at the receiver to allow receiver processes to perform successfully frame synchronization and interleaver functions on the filtered signal. In accordance with an aspect of the invention, such filtering is done using a simple bandpass filter.
We have further recognized that, although such filtering renders the side band signal sufficiently clean to recover frame synchronization and interleaver signals, it nevertheless corrupts the information carried in that signal, thereby making it extremely difficult to recover the information. We deal with this problem by using the filtered version of the received side band signal for frame and interleaver synchronization functions and, after such functions have been performed using a shifted version of the received side band signal to recover such information, all in accordance with an aspect of the invention.
In accordance with another aspect of the invention, the extent of such shifting is determined as a function of the number of signal samples that the filtered signal had to be shifted to obtain frame synchronization, e.g., to locate the beginning of a frame.
These and other aspects of the invention are described in the following detailed description and ensuing claims.